3 Answers
3

Do the latest version, and a quick check with the previous if you really want. There's a post someone did a few months back with graphs showing what happens to various browsers' versions once a new one comes out. It was primarily about Chrome, because of its automatic updating, but he also saw that since 4 Firefox has been doing basically the same thing: as soon as a new version is released, the previous one basically disappears and is no longer a consideration. (Once you factor in the people that are stuck on 3.6, which you're already aware of.)

I test with 3.6 and the latest version. I'm adding in the Android version soon although I don't anticipate there being any big differences there. Unless you're doing HTML5 stuff or something else along those lines you probably won't need to test every version.